Yes, it’s controlled by the size of your large image in Settings, Media. You’ll need to regenerate image thumbnails after changing the size. See resize images across the intranet – you’ll need the Regenerate Thumbnails plugin.
Thanks for this. I think I didn’t phrase my question very well. What I want to do is have the first news item use the small picture size not the large picture size (rather than change to size of the large picture). Can I do that?